The electrically powered passenger vehicle has long been considered one of the most attractive alternatives to vehicles powered by conventional internal combustion engines from the standpoints of overall efficiency, environmental impact and, most recently alternative fuel capability. Many commercial enterprises and private individuals, some under the auspices of the federal government, have proposed various approaches to implementing an electrically powered passenger road vehicle. To date, there have been virtually no such vehicles which have been produced on a large scale.
There have been a number of different approaches to the implementation and control of an electric vehicle. The most attractive approach from the applicant's viewpoint is a vehicle employing a battery bank and an AC induction motor. AC motors are relatively light-weight, inexpensive and easy to manufacture when compared to DC motors. AC induction motors, with no brushes or commutation, are more rugged and reliable than their DC counterparts, and require less maintenance. AC machines can be driven at substantially greater speeds than equivalent power DC motors giving a more attractive power-to-weight ratio. Because AC motors do not generate sparks, they can readily be employed in dusty, explosive and highly humid atmospheres, and at high altitudes. Additionally, AC motors can be liquid cooled if the application so requires.
A prior system utilizing pulse width modulated inverter control of an AC motor in an electric vehicle is disclosed in U.S. Pat. No. 4,316,132, the disclosure of which is incorporated herein by reference. In addition, methods of pulse width modulation control of three-phase AC motors are discussed in an article entitled "Introduction to Pulse Width Modulation Speed Control System for Three-Phase AC Motors", published in "Electronic Components and Applications", Vol. 2, No. 2, Feb. 1980, and in an article entitled "LSI Circuit for AC Motor Speed Control", published in "Electronic Components and Applications", Vol. 2, No. 4, Aug. 1980. The disclosure of these articles is incorporated herein by reference.
Although the known techniques and apparatus for control of AC induction motors, including those disclosed in the application and articles above, have generally proved acceptable for their intended purposes, they have not been satisfactory in all respects.
Accordingly, it is an object of the present invention to provide a method of pulse width modulation for controlling an inverter driving an AC induction motor which provides an average voltage across any two-phase inputs of the motor that varies sinusoidally, which has a minimum of harmonics, which presents a higher effective switching rate to the motor in relation to the inverter switching rate to reduce inverter switching losses, and which minimizes the peak currents flowing through the inverter and AC motor.
It is a further object of the present invention to provide a method of pulse width modulation, as aforesaid, in which the modulation function is relatively simple and can be evaluated quickly and efficiently.
It is a further object of the invention to provide an apparatus which uses pulse width modulation techniques to control an inverter driving an AC induction motor, which apparatus is efficient, is relatively inexpensive to manufacture in production quantities, and is sufficiently rugged and dependable when used in an environment such as a vehicle.
It is a further object of the present invention to provide an apparatus which uses pulse width modulation techniques to control an inverter driving an AC motor, which apparatus includes a battery charger which requires a minimum amount of additional circuitry.
A further object of the present invention is to provide a control apparatus having a battery charger circuit, as aforesaid, in which certain portions of the inverter circuit also function as part of the battery charger circuit.